A convenient modulation arrangement for high speed digital transmission systems known as quadrature amplitude modulation or QAM involves modulating the amplitudes of quadrature-related carriers with digital data channels. Quadrature-related carriers are carriers of the same frequency having a phase angle therebetween of 90 degrees. After amplitude modulation, the carriers are added and the resulting sum is transmitted through a suitable transmission medium. At the receiver, the incoming signal is demodulated by regenerating the carrier and synchronously detecting the incoming signal with the regenerated carrier.
QAM modulation defines a group of signal data points on a two-dimensional signal space diagram having four quadrants. The four quadrants are defined by an inphase or I axis and a quadrature phase or Q axis. The number of signal points in each quadrant is a function of the number of amplitude levels for each modulated carrier.
A problem occurring in QAM transmission systems is phase ambiguity in the regenerated carrier. Phase ambiguity arises when the regenerated carrier skips to a different stable phase relative to the transmitted carrier and can result in a loss of frame synchronization and incorrectly regenerated data bits. Referring to the signal space diagram, the presence of phase ambiguity results in a rotation of the data points by a multiple of a whole quadrant. Accordingly, during phase ambiguity, any transmitted data point in one quadrant will be regenerated as a corresponding data point within one of the three other quadrants.
A prior art technique to resolve phase ambiguity involves the use of differentially encoded data. While differential encoding allows system operation without an absolute carrier reference phase, the bit error rate performance can be worse than that obtainable with nondifferential data encoding. In fact, for many QAM system applications, the degraded bit error rate performance resulting from differential encoding does not meet performance objectives. Furthermore, this unsatisfactory bit error rate can not be significantly improved using easily implementable error correction techniques because of the burst of bit errors resulting from a single transmission error.
Another technique to resolve phase ambiguity in phase shift keying (PSK) satellite systems, as disclosed in U.S. Pat. No. 3,736,507 to Wolejsza, Jr. et al, issued May 29, 1973, involves the transmission of a repetitive, fixed word at the beginning of each transmission burst. This scheme, however, is not suited for some QAM systems, especially terrestrial systems which continuously transmit data, because it can generate spectral tones with high power levels.